Multiple myeloma (MM), which accounts for 1% of cancers and 2% of cancer deaths, is a liquid tumor characterized by expansion of monoclonal plasma cells and immunoglobulin (M protein) secretion. Despite recent advances in treatment, median duration of survival of MM remains three years. Our recent results show that peripheral blood contains increased numbers of endothelial progenitor cells (EPCs) in MM patients. These EPCs express high levels of pro-angiogenic receptor KDR, and display in vitro colony formation, outgrowth, and capillary network formation, which was inhibited after effective thalidomide treatment. Furthermore, EPCs display an extremely skewed (> 90%) pattern of X-chromosome inactivation, whereas lymphocyte and hair follicle specimens from the same patients revealed a random inactivation pattern. Our studies provide the first evidence for clonality in EPCs in MM and suggest that these cells are tumor-related. The cause and consequences of this observation are crucial to study in MM because unknown aspects of MM pathogenesis could be tightly related to endothelial cells. For example, results from such studies could challenge the concept that MM is solely a disease of plasma cells, and might also suggest that neoplastic clone in MM is an earlier progenitor/precursor cell. Clinical evidence for this possibility is the occurrence of idiopathic myelodysplastic syndrome commonly seen in untreated patients, and the success of tandem bone marrow transplants in MM treatment. We hypothesize that EPCs in MM are tumor-related and, similar to MM cells, display clonality and express MM-specific gene expression patterns which are not present in EPCs from controls. As a corollary of this hypothesis, we expect that progressive clinical stages of MM are characterized by an EPC gene expression profile that indicates mechanisms responsible for disease progression. The Specific Aims of this proposal, therefore, are to test the following three hypotheses. 1. Circulating EPCs indicate disease severity and prognosis in MM. Groups of patients stratified for disease severity and controls will be studied by flow cytometry and in vitro colony formation to quantitate EPCs, the results of which will be compared to bone marrow microvascular density, M protein, MM and EPC karyotype, and beta2-microglobulin. 2. Gene expression patterns of circulating EPCs determine the clinical behavior of MM. Mechanisms involved in MM pathogenesis and progression will be examined by comparing MM-specific gene expression patterns in outgrown EPCs from patient groups and from controls. 3. Circulating EPCs display clonality and other genetic similarities to MM cells. The sex chromosome inactivation patterns in outgrown EPCs from peripheral blood and non-endothelial cells will be compared. EPCs and MM cells will be studied for MM-specific chromosomal translocations and for immunoglobulin heavy chain gene rearrangements. The linkage of genetic abnormalities associated with EPCs with the biology of MM will alter the trajectory of the development of new-targeted therapies as well.